The cabal-sort package

If you have a bunch of packages you may want to compile or recompile,
then you need an order of compilation that meets the dependencies.
Given a number of cabal package files,
this program reads all those files
and emits them topologically sorted according to their dependencies.
This way you can compile many packages at once,
say if a very low-level package has changed.

For compiling a couple of packages from their local darcs repositories
in the right order, you may run something like

Problem 1: Given packages A, B, C,
where C depends on B and B depends on A,
and you call

cabal-sort C.cabal A.cabal

then the emitted order of packages may be wrong,
because cabal-sort does not get to know the dependency of C on B.
Even if the order is correct,
B.cabal is missing in the output
and thus the list of cabal files cannot immediately be used
for a sequence of cabal-install runs.

Problem 2: We ignore flags and merge all dependencies.
This may lead to dependency cycles that cannot occur for any flag assignment.

There is a second program called ghc-pkg-dep
that finds recursively all packages that a set of packages depends on.
Duplicates are eliminated and the packages are given topologically sorted,
such that you can use this for recompilation of the packages.

ghc-pkg-dep pkgA-0.1 pkgB-2.3 pkgC-0.1.2

Unfortunately ghc-pkg runs quite slowly.
In order to not get bored you may run the program with --verbose=2 option.
Maybe there is a way to query the complete GHC package database at once.